Interpretive Summary: Slug damage in western Oregon seed-producing fields can be extensive and expensive to control. Weather assosicated slug activity is poorly understood in this environment because nearly all published research of the gray field slug has been conducted in countries or states with weather patterns that differ markedly from that of the Willamette Valley. This study measured the effect of temperature and moisture on slug activity under greenhouse conditions established to mimic those encountered in western Oregon. We found that the initial slug emergence from protected areas of the soil occurred in late summer to early autumn when soil temperatures began to fall. As soil moisture increased, slugs become concentrated near the surface. We found that early season baiting controls slugs easier if adequate moisture from dew activates dry baits although at this time of the season, slugs were vertically dispersed in the soil. As soil temperatures declined, control becames more problematic because slugs were less active and there was apparent weathering effects on the bait.

Technical Abstract:
Slug damage in western Oregon seed-producing fields can be extensive and expensive to control. A lack of data on the effects of temperature and moisture has limited efforts to reduce slug numbers because there was insufficient data on when slugs were most active and likely to be exposed to bait. This study quantified the effect of temperature and moisture on slug activity under greenhouse conditions that were established to mimic those encountered in western Oregon. Under these conditions, initial slug emergence from protected areas of the soil occurred in late summer when soil temperatures declined. As soil moisture increased, slugs become concentrated near the surface. Early season baiting was more effective when adequate moisture from dew activated dry baits although at this time of the season, slugs were vertically dispersed in the soil. As soil temperatures declined, control becames problematic because slugs were less active and there was apparent weathering effects on the bait.